When she came into the room, she smiled warmly at me.

Literal

Room [to-に] entered [toward speaker-てきた] [when-時], she [topic-は] me [to-に] warmly smiled.

にっこり is a 擬態語 specifically for a warm, genuine smile — distinct from にやにや (sly grin) or にたにた (creepy smile). The と after にっこり marks it adverbially.